Method of impurity introduction and

ABSTRACT

A method of introducing an impurity into a wafer surface is provided. The method comprises the steps of: low energy implantation of impurity into a surface of the wafer to generate an implanted dopant layer; and simultaneously removing an implanted surface of the implanted dopant layer to generate a doping profile with controlled areal impurity dosage.

RELATED APPLICATIONS

This application claims priority benefit of U.S. Provisional ApplicationSer. No. 60/911,076, filed Apr. 10, 2007, which is herein incorporatedby reference.

BACKGROUND

1. Field of Invention

The present invention relates to a method of introducing an impurityinto a wafer surface. More particularly, the present invention relatesto a method of introducing an impurity into a wafer surface with lowenergy implantation and controlled surface removal.

2. Description of Related Art

Ion implantation has been a process of choice to introduce dopant intosilicon substrate in advanced CMOS technology. As device continue to bescaled, the need for ultra shallow junction formation becomesincreasingly important for the source/drain area. In the production ofvery small devices, it is necessary to reduce the junction depths. Mostof the work in shallow junction formation has concentrated on applyingconventional ion implantation to form very shallow source/drain forsubmicron CMOS.

A great deal of emphasis has been placed on reducing the implant energy.Many manufacturers of ion implant equipment are pushing their systems toa few keV. Some options for forming ultra shallow junctions includesusing a layer on top of the silicon as a diffusion source, using aplasma immersion doping as an ion source. However, modern ion implantersare often expensive and present many challenges to form ultra shallowjunctions. For example, advanced low energy implanter has to adddecelerators to conventional tool set up in order to achieve the properlow energy range. This addition poses challenges in technical parametersand cost. Plasma immersion doping method has the potential for shallowjunction formation. However, the dopant dosage is hard to control withhigh precision, namely, the uniformity of the profile across the waferis difficult to control. Therefore, a new method of high dose and lowenergy ion implantation is needed.

SUMMARY

The present invention provides a method of introducing an impurity intoa wafer surface, the method comprises the steps of: low energyimplantation of impurity into a surface of the wafer to generate animplanted dopant layer; and simultaneously removing an implanted surfaceof the implanted dopant layer to generate a doping profile with uniformareal impurity dosage.

In general, ion-milling machine does not contain precision voltagecontrol to achieve energy purity or mass selection mechanism to achieveatomic species purity. However, ion-milling can be viewed as a lowenergy implantation mechanism with an additional surface removal by aphysical sputtering mechanism.

By combining implantation and surface removal simultaneously, as will bedemonstrated, the dopant profile and dosage will achieve a steady stateafter the removing of a surface layer with a thickness equals toapproximately two times of the implant range.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIG. 1 is a diagram of the normalized first doping profile for the lowenergy implant when the amount of surface removal is yet negligible; and

FIG. 2 is a diagram of the normalized second doping profile when thesurface layer is being removed at a rate of s.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

Ion-milling machines are used to implant low energy and high dosage ionsinto the wafer surface. The basic ion-milling mechanism is to ionize theinput gas in an ionization chamber and accelerate the ions through anelectric field to attain certain energy. The energetic beam, containingions and neutrals, impinges on the surface causing simultaneoussputtering as well as shallow implantation. The low cost ion-millingdoes not carry out ion separation or atomic separation. As a result theincident beam is a mixture of ion and neutrals with an energydistribution. In the present embodiment, the incident beam may containboth dopant gas and carrier gas in the form of ion and neutrals with anenergy distribution, and the dosage of dopant in the wafer surface ismodulated by the partial pressure of a mixture of dopant gas and carriergas. The carrier gas is used to modify an atomic density of theimplanted layer, a sputtering yield or a surface etching rate of theremoval process. By combining the implanting and sputter effect ofion-milling, a uniform flat box profile can be obtained. In otherembodiment, the forming steps can be performed by a plasma generatingequipment with proper implant and removal characteristics, and theremoving process can also be made by reactive ion etching (RIE) process.

Please refer to FIG. 1, a normalized first doping profile, G(x), for thelow energy implant when the amount of surface removal is yet negligible.The horizontal axis is the linear distance from the substrate surface,and the vertical axis is the dopant concentration in log scale. G(x) canbe expressed as follow,

$\begin{matrix}{{G(x)} = {\int_{0}^{E_{0}}{{D(E)}{f\left( {x,E} \right)}{E}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

x is a distance from the wafer surface, E is the ion energy, D(E) is theenergy distribution of the incoming dopant flux (with carrier gascomponent, if used, excluded), f(x, E) is the normalized dopantdistribution profile at a monotonic energy, E, and E₀ is an upper limitof the energy distribution of the ion beam. The implant depth, r, can becontrolled to within 10 nm when the accelerating voltage of the ion beamis less than 1 KeV.

The dopant profile, G(x), probably does not have a simple Gaussiandistribution generally used to model a mono energetic implant beam. Theactual profile will depend on the energy distribution of the incidentflux. FIG. 1, for illustration purpose, shows double dopant peaks. Theone peak further into the surface could be due to ions that gained fullaccelerating potential. The shallower peak could be due to neutrals thathad gained energy partially through the accelerating field beforeneutralization occurs.

FIG. 2 shows that as the surface layer is being removed, at a rate “s”,the implant profile, G₀, moves through space and are shown as G₁, G₂,and finally G_(T). With “sT” amount of surface layer removed. In thecase of ion-milling “sT” is the amount of surface layer sputtered. Thesecond doping profile, i.e. the concentration profile 202 isproportional to incident flux, J, and is the result of sputtering over aperiod of time, T, where:

$\begin{matrix}{{{C\left( {x,T} \right)} = {\int_{0}^{T}{{{JG}\left( {x - {st}} \right)}{t}}}},{s = {{JY}\text{/}\rho}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where

s is the rate of surface removal

Y is the effective sputtering yield with carrier gas accounted

ρ is the effective atomic density

x is the distance from the original surface

T is the time

J is the incident flux

C(x, T) is the second doping profile, i.e. the dopant concentration fromthe original surface at time T

G₀(x), G₁(x), G₂(X), G_(T)(x) are the instantaneous dopant profile att=0, 1, 2, and T, respectively. The following coordinate transformationfixes the moving surface to zero:

u=x−st

du=−sdt

or

dt=−du/s

Eq. 2 transforms to

$\begin{matrix}{\begin{matrix}\left. {{C\left( {v,T} \right)} = {\int_{u({({t = 0})}}^{u{({t = T})}}{{- \left( {J\text{/}s} \right)}{G(u)}{u}}}} \right) \\{= {\int_{u({({t = T})}}^{u{({t = 0})}}{\left( {J\text{/}s} \right){G(u)}{u}}}}\end{matrix}{{u\left( {t = T} \right)} = {{x - {sT}} = v}}{{u\left( {t = 0} \right)} = {x = {v + {sT}}}}} & \left( {{{Eq}.\mspace{14mu} 2}A} \right)\end{matrix}$

The variable, ν, is the distance from the etched surface at time, T. Eq.2A becomes;

$\begin{matrix}\begin{matrix}\left. {{C\left( {v,T} \right)} = {J\text{/}s{\int_{v}^{v + {sT}}{{G(u)}{u}}}}} \right) \\{= {J\text{/}{s\left\lbrack {{\int_{0}^{v + {sT}}{{G(u)}{u}}} - {\int_{0}^{v}{{G(u)}{u}}}} \right\rbrack}}}\end{matrix} & \left( {{{Eq}.\mspace{14mu} 2}B} \right)\end{matrix}$

In case the amount removed, sT, is very large or even just larger thanthe implant depth,

∫₀^(v + sT)G(u)u ≅ 1.

Eq. 2B becomes

$\begin{matrix}\begin{matrix}{{C(v)} = {J\text{/}{s\left\lbrack {1 - {\int_{0}^{v}{{G(u)}{u}}}} \right\rbrack}}} \\{= {\rho \text{/}{Y\left\lbrack {1 - {\int_{0}^{v}{{G(u)}{u}}}} \right\rbrack}}}\end{matrix} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Eq. 3 reveals that the dosage is independent of the time T, the fluxdensity J, or the removal rate s. In short, due to the convolution ofthe instantaneous profiles over time T, after the removal of sT amountof material (sT is greater than or equal to r, the implant depth) fromthe wafer surface, the second doping profile 202 becomes a relativelyflat profile down to r, typically a range of less than 20 nm. Therefore,after sputtering through roughly a distance, r, the profile approachessteady state and approximates a classic box profile. As illustratedqualitatively by FIG. 2, if the implant depth is about 5 nm to 20 nm,then by removing 5 nm to 20 nm off the wafer surface, a flat box profileof 5 nm to 20 nm can be formed in the wafer substrate. This profile willbe maintained even if the etch/implant process were to continue further.Due to simultaneous implant and surface removal, the final profileapproaches a steady state as a function of depth into silicon and isuniform across the wafer after the surface layer removed is greater than“r”. The qualitative explanation above can be concisely formulated bythe following mathematical equations.

In addition, an important feature in shallow junction implantation isthe ability to control dose concentration. The total dose, D, remainingin the Si in the steady state can be calculated as follows:

$\begin{matrix}\begin{matrix}{D = {\int_{0}^{\infty}{{C\left( {v,{T = \infty}} \right)}{v}}}} \\{= {J\text{/}s{\int_{0}^{\infty}\left( {\left( {\int_{v}^{\infty}{{G(u)}{u}}} \right){v}} \right.}}} \\{= {\rho \text{/}Y{\int_{0}^{\infty}{{{vG}(v)}{v}}}}}\end{matrix} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Where the projected implant range,

Rp = ∫₀^(∞)vG(v)v.

Therefore, the total implant incorporated dosage is;

$\begin{matrix}{D = {\frac{\rho}{Y}{Rp}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

Eq. 5 indicates that the dosage of the ions is independent of fluxdensity and is only a function of density, sputtering yield andprojected range, ρ, Y and Rp respectively.

In conclusion, the embodiment of the present invention is a method ofimplanting a dose of ions into a wafer surface by the use of anion-milling machine or other energetic sources. The ion-milling machineintroduces an ultra shallow ion implantation in the wafer substratewhile sputtering the wafer surface to achieve uniform doping profile.Other controlled surface removal method, such as RIE can also apply. Byusing ion-milling machines as low energy high dose implanters, cost ofequipment can be significantly reduced while dose uniformity can beobtained independent of purity in the ion beam.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the source of the energeticparticle and the surface removal method of the present invention withoutdeparting from the scope or spirit of the invention. In view of theforegoing, it is intended that the present invention cover modificationsand variations of this invention provided they fall within the scope ofthe following claims and their equivalents.

1. A method of introducing an impurity into a wafer surface, the methodcomprises the steps of: low energy implantation of impurity into asurface of the wafer to generate an implanted dopant layer; andsimultaneously removing the surface of the implanted dopant layer togenerate a doping profile with a controlled areal impurity dosage. 2.The method of claim 1, wherein the implantation step is performed by anion-milling machine.
 3. The method of claim 1, wherein the implantationstep is performed by a plasma generating equipment with proper implantand removal characteristics.
 4. The method of claim 1, wherein thesurface removal process is performed by a sputtering, ion milling orreactive ion etching (RIE) process.
 5. The method in claim 1, whereinthe dosage of the impurity incorporated in the wafer surface becomesindependent of an incident flux of the impurity and its total integrateddosage.
 6. The method in claim 5, wherein the dosage of dopant in thewafer surface is modulated by the partial pressure of a mixture ofdopant gas and carrier gas.
 7. The method in claim 6, wherein thecarrier gas is used to modify the atomic density of the implanted dopantlayer, the sputtering yield or the surface etching rate of the removalprocess.
 8. The method of claim 1, wherein the first doping profile is:G(x) = ∫₀^(E₀)D(E)f(x, E)E; where x is a distance from the wafersurface, E is a ion energy, G(x) is the first doping profile, D(E) is anenergy distribution of an ion beam generated by the ion-milling, f(x, E)is a dopant distribution at E, and E0 is an upper limit of the energydistribution of the ion beam.
 9. The method of claim 1, wherein thesecond doping profile is: .C(x, T) = ∫₀^(T)JG(x − st)t; where C(x)is the second doping profile, s is the rate of removing the wafersurface, J is the incident flux, x is the distance from the originalsurface, t is a removing time and T is the period of time to remove thewafer surface.
 10. The method of claim 1, wherein the wafer surface isremoved by a total amount of 5-20 nm.
 11. The method of claim 1, whereinthe second doping profile has an implant range of less than 20 nm.